This invention relates to a device for temporarily sealing a pipe, as for example the production tubing of an oil well.
Devices of this kind are known and are sometimes referred to as "subs". "Subs" are usually used in conjunction with plugs for production tubing; the subs are normally closed but, when open, provide a passage for the flow of oil through the plug, thus enabling a well to start or recommence production without requiring removal of the plug.
"Subs" which are openable upon the application thereto of pressure by pumping oil or mud down the production tubing are known as "pump out subs".
A previously known "pump out sub" comprises a plunger held by shear pins in a position in which it seals an opening through which oil may pass up the production tubing. The shear pins may be sheared by the pressure of the pumped fluid so that the plunger drops away from the sealing position.
A second previously known "pump out sub" comprises a disc which is sheared from the sub when a sufficient pressure of fluid is applied.
These known "pump out subs" have the disadvantage that deposition of debris, such as sand or shale, will result in formation of a plug of debris on the plunger or disc when fluid is pumped under pressure onto it. In consequence greater pressure must be applied before the pins or disc will shear than if the plug of debris were not present. Also the shear parts may be prematurely sheared by jarring the plug when it is set down inside the tubing by wireline.
A further disadvantage of the aforesaid "pump out subs" is the high pressure differential necessary to effect shearing. Thus for shear disc holding a well pressure of 2000 psi it is usually necessary to apply at least 3000 psi to produce shearing; this high pressure differential of 1000 psi necessitates the use of well Christmas trees capable of coping with the high pressures required for sub opening, and imposes restrictions on the use of subs in wells in which the Christmas trees are rigged to cope with relatively low pressures.